Adam
Frank is a professor of astrophysics at the University
of Rochester. His work has appeared in Scientific
American, The New York Times, and NPR. He is the author
of Light of the Stars: Alien Worlds and the Fate of the
Earth.

One day last year, I came
to GISS with a far-out proposal. In my work as an astrophysicist,
I'd begun researching global warming from an "astrobiological
perspective." That meant asking whether any industrial civilization
that rises on any planet will, through their own activity, trigger
their own version of a climate shift.

I was visiting GISS that
day hoping to gain some climate science insights and, perhaps,
collaborators. That's how I ended up in Gavin's office.

Just as I was revving up my pitch, Gavin stopped me in my tracks.

"Wait a second," he
said. "How do you know we're the only time there's been a
civilization on our own planet?"

It took me a few seconds
to pick my jaw off the floor.

I had certainly come into
Gavin's office prepared for eye rolls at the mention of "exo-civilizations."
But the civilizations he was asking about would have existed many
millions of years ago.

Though neither of us
could see it at that moment, Gavin's penetrating question opened a
window not just onto Earth's past, but also onto our own future.

We're used to imagining extinct civilizations in terms of the sunken
statues and subterranean ruins. These kinds of artifacts of previous
societies are fine if you're only interested in timescales of a few
thousands of years.

But once you roll the
clock back to tens of millions or hundreds of millions of years,
things get more complicated.

When it comes to direct evidence of an industrial civilization -
things like cities, factories, and roads - the geologic record
doesn't go back past what's called the
Quaternary period 2.6 million years
ago.

For example, the oldest
large-scale stretch of ancient surface lies in the
Negev Desert.

It's "just" 1.8 million
years old - older surfaces are mostly visible in cross section via
something like a cliff face or rock cuts. Go back much farther than
the Quaternary and everything has been turned over and crushed to
dust.

And, if we're going back this far, we're not talking about human
civilizations anymore.

Homo sapiens
didn't make their appearance on the planet until just 300,000 years
or so ago. That means the question shifts to other species, which is
why Gavin called the idea the
Silurian hypothesis, after an old
Dr. Who episode with intelligent reptiles.

So,

could researchers find clear evidence that an ancient species
built a relatively short-lived industrial civilization long before
our own?

Perhaps, for example,
some early mammal rose briefly to civilization building during the
Paleocene epoch about 60 million years ago.

There are fossils, of
course. But the fraction of life that gets fossilized is always
minuscule and varies a lot depending on time and habitat.

It would be easy,
therefore, to miss an industrial civilization that only lasted
100,000 years - which would be 500 times longer than our industrial
civilization has made it so far.

Given that all direct evidence would be long gone after many
millions of years, what kinds of evidence might then still exist?

The best way to answer
this question is to figure out what evidence we'd leave behind if
human civilization collapsed at its current stage of development.

Now that our industrial civilization has truly gone global,
humanity's collective activity is laying down a variety of traces
that will be detectable by scientists 100 million years in the
future.

The extensive use of
fertilizer, for example, keeps 7 billion people fed, but it also
means we're redirecting the planet's flows of nitrogen into food
production. Future researchers should see this in characteristics of
nitrogen showing up in sediments from our era.

Likewise our relentless
hunger for the rare-Earth elements used in electronic gizmos. Far
more of these atoms are now wandering around the planet's surface
because of us than would otherwise be the case. They might also show
up in future sediments, too.

Even our creation, and
use, of synthetic steroids has now become so pervasive that
it too may be detectable in geologic strata 10 million years from
now.

And then there's
all that plastic. Studies have
shown increasing amounts of plastic "marine litter" are being
deposited on the seafloor everywhere from coastal areas to deep
basins and even in the Arctic.

Wind, sun, and waves
grind down large-scale plastic artifacts, leaving the seas full of
microscopic plastic particles that will eventually rain down on the
ocean floor, creating a layer that could persist for geological
timescales.

The big question is how long any of these traces of our civilization
will last...

In our study, we found
each had the possibility of making it into future sediments.
Ironically, however, the most promising marker of humanity's
presence as an advanced civilization is a by-product of one activity
that may threaten it most.

When we burn fossil fuels, we're releasing carbon back into the
atmosphere that was once part of living tissues. This ancient carbon
is depleted in one of that element's three naturally occurring
varieties, or isotopes. The more fossil fuels we burn, the more the
balance of these
carbon isotopes shifts.

Atmospheric scientists
call this shift the
Suess effect, and the change in
isotopic ratios of carbon due to fossil-fuel use is easy to see over
the last century. Increases in temperature also leave isotopic
signals.

These shifts should be
apparent to any future scientist who chemically analyzes exposed
layers of rock from our era.

Along with these spikes,
this
Anthropocene layer might also hold
brief peaks in,

nitrogen

plastic
nanoparticles

even synthetic
steroids

So if these are traces
our civilization is bound to leave to the future, might the same
"signals" exist right now in rocks just waiting to tell us of
civilizations long gone?

Fifty-six million years ago, Earth passed through the
Paleocene-Eocene Thermal Maximum (PETM).

During the PETM, the
planet's average temperature climbed as high as 15º Fahrenheit
(-9.44ºC) above what we experience today. It was a world almost
without ice, as typical summer temperatures at the poles reached
close to a balmy 70º Fahrenheit (21ºC).

Looking at the isotopic
record from the PETM, scientists see both carbon and oxygen isotope
ratios spiking in exactly the way we expect to see in the
Anthropocene record.

There are also other
events like the PETM in the Earth's history that show traces like
our hypothetical Anthropocene signal.

These include an event a
few million years after the PETM dubbed the Eocene Layers of
Mysterious Origin, and massive events in the Cretaceous that
left the
ocean without oxygen for many
millennia (or even longer).

Are these events indications of previous nonhuman industrial
civilizations? Almost certainly not.

While there is evidence
that the PETM may have been driven by a massive release of buried
fossil carbon into the air, it's the timescale of these changes that
matter.

The PETM's isotope spikes
rise and fall over a few hundred thousand years. But what makes the
Anthropocene so remarkable in terms of Earth's history is the speed
at which we're dumping fossil carbon into the atmosphere.

There have been
geological periods where Earth's CO2 has been as high or
higher than today, but never before in the planet's
multibillion-year history has so much buried carbon been dumped back
into the atmosphere so quickly.

So the isotopic spikes
we do see in the geologic record may not be spiky enough to fit the
Silurian hypothesis's bill.

But there is a
conundrum here. If an earlier
species' industrial activity is short-lived, we might not be able to
easily see it. The PETM's spikes mostly show us the Earth's
timescales for responding to whatever caused it, not necessarily the
timescale of the cause.

So it might take both
dedicated and novel detection methods to find evidence of a truly
short-lived event in ancient sediments. In other words, if you're
not explicitly looking for it, you might not see it.

That recognition was,
perhaps, the most concrete conclusion of our study.

It's not often that you write a paper proposing a hypothesis that
you don't support. Gavin and I don't believe the Earth once hosted a
50-million-year-old Paleocene civilization.

But by asking if we could
"see" truly ancient industrial civilizations, we were forced to ask
about the generic kinds of impacts any civilization might have on a
planet.

That's exactly what the
astrobiological perspective on climate change is all about.

Civilization building
means harvesting energy from the planet to do work (i.e., the work
of civilization building).

Once the civilization
reaches truly planetary scales, there has to be some feedback on the
coupled planetary systems that gave it birth (air, water, rock).

This will be particularly
true for young civilizations like ours still climbing up the ladder
of technological capacity. There is, in other words, no free lunch.
While some energy sources will have lower impact - say solar vs.
fossil fuels - you can't power a global civilization without some
degree of impact on the planet.

Once you realize, through climate change, the need to find
lower-impact energy sources, the less impact you will leave.

So the more sustainable
your civilization becomes, the smaller the signal you'll leave for
future generations.

In addition, our work also opened up the speculative possibility
that some planets might have fossil-fuel-driven cycles of
civilization building and collapse.

If a civilization uses
fossil fuels, the climate change they trigger can lead to a large
decrease in ocean oxygen levels. These low oxygen levels (called
ocean anoxia) help trigger the
conditions needed for making fossil fuels like oil and coal in the
first place.

In this way, a
civilization and its demise might sow the seed for new civilizations
in the future.

By asking about civilizations lost in deep time, we're also asking
about the possibility for universal rules guiding the evolution of
all biospheres in all their creative potential, including the
emergence of civilizations.

Even without
pickup-driving Paleocenians, we're only now learning to see how rich
that potential might be...